1. Field of the Invention
The present invention relates to novel membrane electrode assemblies, improved membranes for use in such membrane electrode assemblies, and fuel cells employing such membrane electrode assemblies.
2. Brief Description of the Related Art
Fuel cells are electrochemical cells in which a free energy change resulting from a fuel oxidation reaction is converted into electrical energy. Fuel cells are attractive electrical power sources, due to their higher energy efficiency and environmental compatibility compared to the internal combustion engine. The most well-known fuel cells are those using a gaseous fuel (such as hydrogen) with a gaseous oxidant (usually pure oxygen or atmospheric oxygen), and those fuel cells using direct feed organic fuels such as methanol. In contrast to batteries, which must be recharged, electrical energy from fuel cells can be produced for as long as the fuels, e.g., methanol or hydrogen, and oxidant, are supplied. Thus, a considerable interest exists in the design of improved fuel cells to fill future energy needs.
While a number of different types of electrochemical cells have been employed in the manufacture of fuel cells, arguably ion exchange membrane (IEM) cells have received the most attention. An IEM cell typically employs a membrane comprising an ion-exchange polymer. This ion-exchange polymer membrane serves as a physical separator between the anode and cathode, while also serving as an electrolyte. IEM cells can be operated as electrolytic cells for the production of electrochemical products, or operated as fuel cells for the production of electrical energy.
In some IEM cells, a cation exchange membrane is used wherein protons are transported across the membrane as the cell is operated. Such cells are often referred to as proton exchange membrane (PEM) cells. For example, in a cell employing the hydrogen/oxygen couple, hydrogen molecules (fuel) at the anode are oxidized donating electrons to the anode, while at the cathode the oxygen (oxidant) is reduced accepting electrons from the cathode. The H+ions (protons) formed at the anode migrate through the membrane to the cathode and combine with oxygen to form water. In many fuel cells, the anode and/or cathode comprises a layer of electrically conductive, catalytically active particles (usually in a polymeric binder) on the proton exchange membrane. The resulting structure (sometimes also including current collectors) is referred to as a membrane electrode assembly (MEA).
In one approach to the construction of an ion exchange membrane, perfluornated sulfonic acid polymers such as Nafion(copyright) (and other ion exchange materials) are incorporated into films, for example porous polytetrafluoroethylene (PTFE), to form composite membranes,:as described for example in U.S. Pat. No. 5,082,472, to Mallouk, et al.; JP Laid-Open Pat. Application Nos. 62-240627, 62-280230, and 62-280231; U.S. Pat. No. 5,094,895 to Branca, U.S. Pat. No. 5,183,545 to Branca et al.; and U.S. Pat. No. 5,547,551 to Bahar, et al. (each of the foregoing references being incorporated herein in their entirety).
In another approach to construction of an ion exchange membrane, a composite membrane is prepared, for example, by precipitation of a water-insoluble, inorganic conductor such as zirconium hydrogen phosphate into a porous Nafion(copyright) membrane (See,. e.g., CT/US96/03804 to Grot, et al.). or incorporation of phosphotungstic acid into a Nafion(copyright) membrane (See, e.g.,., S. Malhotra, et al., in xe2x80x9cJournal of the Electrochemical Society,xe2x80x9d Vol. 144, No. 2, L23-L26, 1997xe2x80x94although the resulting membrane was said to demonstrate high conductivity at elevated temperature, the composite membrane lacked sufficient strength at reduced thickness for hydrogen fuel cell applications).
Fuel cells that employ IEMs and direct organic fuels such as methanol frequently suffer from so-called xe2x80x9ccrossoverxe2x80x9d of fuel through the membrane. The term xe2x80x9ccrossoverxe2x80x9d refers to the undesirable transport of fuel through the membrane from the fuel electrode, or anode, side to the oxygen electrode, or cathode side of the fuel cell. After having been transported across the membrane, the fuel will either evaporate into the circulating oxygen stream or react with the oxygen at the oxygen electrode. Fuel crossover diminishes cell performance for two primary reasons. Firstly, the transported fuel cannot react electrochemically to produce useful energy, and therefore contributes directly to a loss of fuel efficiency (effectively a fuel leak). Secondly, the transported fuel interacts with the cathode, i.e., the oxygen electrode, and lowers its operating potential and hence the overall cell voltage. The reduction of cell voltage lowers specific cell power output, and also reduces the overall efficiency.
Fuel cells that employ IEMs and hydrogen as a fuel also suffer from disadvantages. Certainly, the difficulty of on-board storage and refueling of hydrogen is a major concern in the application of hydrogen fuel cells in vehicles. One approach for surmounting this obstacle has been to utilize the hydrogen fuel obtained through steam reforming of gasoline. Unfortunately, hydrogen fuel from steam reforming of gasoline usually contains a trace amount of carbon monoxide, which results in severe poisoning of anode catalysts. Operating the fuel cell at high temperature can effectively alleviate the carbon monoxide poisoning of anode catalysts. However, at elevated temperature, membranes comprising perfluorinated sulfonic acid polymers such a Nafion(copyright) quickly lose ionic conductivity at ambient pressure due to dehydration. Operation at high temperatures with such membranes thus requires that the cells be pressurized.
One particularly useful group of cation-exchange membrane materials for PEM cells is perfluorinated sulfonic acid polymers such as Nafion(copyright), available from E.I. duPont de Nemours and Co. Such cation-exchange polymers have good conductivity and chemical and thermal resistance, which provide long service life before replacement. However, increased proton conductivity is desired for some applications, particularly for fuel cells, which operate at high current densities.
PEMs must have enough strength, minimum fuel crossover, and high ionic conductance at elevated temperature to be useful in fuel cell applications using hydrogen fuel from partial oxidation or steam reforming of hydrocarbons or other sources. Membrane thickness has been reduced in an effort to improve conductance. However, reduction in thickness results in insufficient membrane strength, necessitating use of additional reinforcing materials, and an increase in crossover. For example, pure Nafion(copyright) membranes have not provided sufficient strength at reduced thicknesses. To increase the strength additional reinforced materials are needed.
PEMS also require effective catalysts associated with the membranes to provide for reactivity with the fuel source and resulting products of catalysis. Typically, a catalyst layer is applied to the membrane using, for example, a combination of temperature, pressure, and perhaps an adhesive. Such layered structure may be placed between two porous substrates.
Most recently, an alternative low-platinum-loading catalyst layer structure has been developed by Wilson at LANL (M. S. Wilson, U.S. Pat. Nos., 5,211,984 and 5,234,777 (1993)) and Grot (U.S. Pat. 5,330,860) to make membrane electrode assemblies. In this structure, recast ionomer (Nafion(copyright)) is used instead of PTFE to bind the catalyst layer structure together, and the low-loading catalyst layer is applied to the membrane, rather than to the gas diffusion structure. Such (PTFE-free) layers have been described as xe2x80x9cthin-filmxe2x80x9d catalyst layers, because the high performance is obtained with a very low catalyst loading (0.12-0.16 mg Pt/cm2) in a thin layer ( less than 10 xcexcm thick). By virtue of their thinness and the high ionomer contents achievable with these catalyst layers, high catalyst utilizations are obtained and the continuity and integrity of the catalyst layer/membrane interface is greatly improved compared with the structures prepared by hot pressing catalysts that are bonded to the gas diffusion layers on to the membrane.
There accordingly remains a need for a membrane capable of maintaining high conductivity at elevated temperature, and which will enable use of PEM fuel cells for vehicle applications. There remains a further need for a membrane that maintains functionality in methanol/hydrogen fuel cells in particular when the fuel contains trace carbon monoxide, as for example, produced during a steam reforming process. There remains a further need for a membrane that operates at a temperature high enough to boil water for use in fuel processing and provide high quality waste heat for on-site space heating use. There further remains a need for a membrane exhibiting sufficient strength at reduced thicknesses, high conductance at elevated temperature, and minimum fuel crossover for hydrogen fuel cell applications.
The present provides a MEA comprising a composite membrane structure having a porous polymeric matrix, ionically conductive solid dispersed in the polymeric matrix and an ionomeric binder, that is flanked by a anode and cathode catalytic layer. A preferred anode catalytic layer of the present invention comprises an oxidizing catalyst composition in intimate contact with carbon powder, an ionically conductive solid, and an ionomeric binder positioned to bind the ionically conductive solid to the oxidizing catalyst composition. A preferred cathode catalytic layer of the present invention comprises a reducing catalyst composition in intimate contact with carbon powder, an ionically conductive solid, and an ionomeric binder positioned to bind the ionicaly conductive solid to the reducing catalyst composition.
It has been unexpectedly found that the incorporation of effective amounts of an ionically conductive solid into both the cathode and anode layers, as well as the composite membrane structure, may greatly improve the overall performance of the MEA, in particular with regard to voltage drop across the assembly. It has further been unexpectedly found that the performance of a MEA comprising a PTFE membrane imbued with ionically conductive solids, can be greatly enhanced by heat treatment of the ionically conductive solid-imbued PTFE membrane. While not be bound thereby, applicants have hypothesized that such improvement is due to a marked reduction in cross-over.
Formulation of the oxidizing and reducing catalyst compositions found useful in the present invention is dependent on the type of oxidant utilized and fuel employed. For example, if hydrogen is used as a fuel, the catalyst composition should be active as a hydrogen oxidation catalyst (such as a 40 wt % platinum/ruthenium alloy supported on the surface of carbon powder), and if oxygen is used as the oxidizing agent on the cathode portion of the MEA, the cathode catalyst should be active as an oxygen reduction catalyst (such as platinum/chromium/cobalt alloy supported on the surface of carbon powder).
In one embodiment of the present invention there is disclosed a membrane electrode assembly (MEA) comprising: (a) a composite membrane having a first major surface area and a second major surface area comprising: (1) a membrane layer comprising an ionically conductive solid and an ionomeric binder; and (2) at least one protective layer disposed adjacent to the membrane layer comprising an an ionically conductive solid and ionomeric binder, and optionally hygroscopic fine powder; (b) an anode comprising an oxidizing catalyst adjacent said first major surface area of said composite membrane; and (c) a cathode comprising a reducing catalyst adjacent said second major surface area of said composite membrane.
In another embodiment of the present invention there is disclosed a membrane electrode assembly (MEA) comprising a composite membrane having a first major surface area and a second major surface area comprising:
1) a porous polymeric matrix containing ionically conductive solid and ionomeric binder;
2) at least one protective layer disposed adjacent to the porous polymeric matrix composite membrane comprising an ionomeric binder and an ionically conductive solid;
and an anode comprising an oxidizing catalyst adjacent said first major surface area of said composite membrane, and a cathode comprising a reducing catalyst adjacent said second major surface area of said composite membrane.
In another embodiment of the present invention there is disclosed a process for fabricating a MEA comprising: (a) obtaining a composite membrane having a first major surface area and a second major surface area comprising: (1) a porous polymeric matrix containing ionically conductive solid and an ionomeric binder, and (2) at least one protective layer disposed adjacent to the porous polymeric matrix composite membrane comprising an ionomeric binder and an ionically conductive solid; (b) spraying a mixture of oxidizing catalyst, ionomeric binder and ionically conductive solid in a solvent on said first major surface area; and (c) spraying a mixture of reducing catalyst, ionomeric binder and ionically conductive solid in a solvent on said second major surface area.
And in yet another embodiment of the present invention there is disclosed a process for fabricating a membrane electrode assembly (MEA) comprising: (a) obtaining a composite membrane having a first major surface area and a second major surface area comprising: (1) a membrane layer containing ionically conductive solid and an ionomeric binder, (2) at least one protective layer disposed adjacent to the membrane layer comprising an ionomeric binder and an ionically conductive solid, and optionally a hygroscopic fine powder; (b) spraying a mixture of oxidizing catalyst, ionomeric binder and ionically conductive solid in a solvent on said first major surface area; (c) spraying a mixture of reducing catalyst, ionomeric binder and ionically conductive solid in a solvent on said second major surface area.
And in yet another embodiment of the present invention there is disclosed a process for fabricating a MEA comprising: (a) obtaining a composite membrane having a first major surface area and a second major surface area comprising: (1) a porous polymeric matrix containing ionically conductive solid and an ionomeric binder, and (2) at least one protective layer disposed adjacent to the porous polymeric matrix composite membrane comprising an ionomeric binder and an ionically conductive solid; (b) applying a mixture of oxidizing catalyst, ionomeric binder and ionically conductive solid in a solvent on said first major surface area; and (c) applying a mixture of reducing catalyst, ionomeric binder and ionically conductive solid in a solvent on said second major surface area.
And yet in other process embodiment of the present invention there is disclosed a process of fabricating a membrane electrode assembly (MEA) comprising: (a). obtaining a membrane having a first major surface area and a second major surface area; (b) applying a solvent comprising an oxidizing catalyst, inomeric binder, and ionically conductive solid in a solvent of said first major surface area; and (c) applying a mixture of reducing catalyst, ionomeric binder, and ionically conductive solid on said second major surface area.
The above-discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings.